Sensors, in particular gas sensors, have been utilized for many years in several industries (e.g., in furnaces and other enclosures, in exhaust streams such as flues, exhaust conduits, and the like, and in other areas). For example, the automotive industry has used exhaust gas sensors in automotive vehicles to sense the composition of exhaust gases, for example, oxygen. A sensor is used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion.
One type of sensor uses an ionically conductive solid electrolyte between porous electrodes. For oxygen detection, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, i.e., reference gas, is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine's exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (“emf”) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle may consist of an ionically conductive solid electrolyte material disposed between a porous electrode with a porous protective overcoat exposed to exhaust gases (“sensing electrode”), and a porous electrode exposed to the partial pressure of a known gas “reference electrode”. Sensors used in automotive applications may use a yttria stabilized zirconia electrolyte with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine's exhaust. Also, a sensor may have a ceramic heater attached to help maintain the sensor's ionic conductivity at low exhaust temperatures. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:
  E  =            (                                    -            R                    ⁢                                          ⁢          T                          4          ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )                            where:        E=electromotive force        R=universal gas constant        F=Faraday constant        T=absolute temperature of the gas        PO2ref=oxygen partial pressure of the reference gas        PO2=oxygen partial pressure of the exhaust gasDue to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating in fuel-rich or fuel-lean conditions, without quantifying the actual air-to-fuel ratio of the exhaust mixture.        
In planar sensors, leads embedded in the sensor element may be used to electrically communicate with the electrodes and/or heater. A wire that is external to the sensor element may be in electrical communication with the leads through the use of via-holes. These via-holes are filled with a precious metal paste that when fired connects the leads to contact pads located on the external surface of the sensor element. In a subsequent step, a connector joined to the wire may be connected to the contact pads. As sensor elements employ more leads, however, this interconnection becomes increasingly complex. Moreover, additional leads relate to the use of more via-holes, which further relate to the use of more precious metal used to fill additional via-holes and make the associated contact pads.
There thus remains a need for sensor elements having an improved electrical interconnection.